1. Field of the Invention
The present invention relates generally to wound dressings such as are useful in treatment of exuding wounds both in first aid as well as longer term treatment situations, particularly where the dressing imparts antimicrobial activity and better than normal accelerated healing to the wound site.
2. Description of the Prior Art
Flexible cellular polyurethane foams are well known and widely used in a variety of industries. These uses range from such things as furniture cushioning and carpet underlayment to cosmetic pads and applicators. Cellular urethane foams have traditionally been prepared using TDI (toluene diisocyanate) polyisocyanate components. In similar fashion, MDI (methylene diisocyanate) has been used to make high density open cell flexible polyurethane foams. These starting materials can be used to manufacture dense foams, i.e., greater than 4.5 lbs./ft3. Various references in the prior art teach the use of resiliently compressible foamed plastic and similar synthetic materials for wound dressings. Such materials can permit comfortable application of pressure even on curved or other non-planar body surfaces.
While the material properties of a foamed plastic may not be critical in the case of cushioning, insulating or packing materials, they are highly critical for purposes of an adequate wound treatment-dressing. For example, it is important or desirable with wound dressings of the type under consideration to have surface properties which for the wound-facing surfaces permit easy flow of moisture into the dressing while avoiding sticking to the wound, and for the outer surface of the dressing to provide at least to a certain extent, a water resistant, breathable barrier. However a number of problems arise in connection with the provision of satisfactory material properties in the context of foam materials of the type under consideration. These problems are discussed in the patent literature and in the relevant technical literature.
For example, British Patent 1417962 describes the use of a non-reticulated polyurethane foam which is modified at the wound-facing inner surface, by application of heat and pressure, to give a layer of collapsed cells, which layer is soft, pliant and facilitates flow of moisture from the wound into the body of the foam material.
A further refinement of such polyurethane foams, involving the use of a body of open-celled hydrophilic foam, is described in WO 92/13576. The addition of an alginate composition to the foam is said to raise the absorptive capacity of the foam and facilitates flow of moisture at a relatively high rate suited to use with very moist wounds.
U.S. Pat. No. 3,903,232 discloses hydrophilic cross-linked polyurethane foams, which are said to be useful for the absorption of body fluids and may be used for external body cleaning, for internal body usage, and as absorptive products such as diapers. The foams are prepared by reacting particular isocyanate-capped polyoxyethylene polyols having an isocyanate functionality greater than 2 with large amounts of an aqueous reactant, preferably water.
EP-A-0335669 discloses a hydrophilic foam composition comprising the “in situ” reaction product of an isocyanate-capped polyether prepolymer, a hydrophilic agent capable of absorbing water, an adjuvant comprising an alcohol, a wetting agent and water. One application which is proposed for the foam composition is in the manufacture of wound dressings. The composition is said to carry the adjuvant releasably, so that at least a portion of the adjuvant is released into an external liquid (e.g. wound exudate) with which the foam composition comes into contact.
U.S. Pat. No. 4,728,323, teaches a method of manufacturing a wound dressing which is comprised of a “substrate” coated with an anticmicrobially effective film of a silver salt. The film is preferably deposited on the substrate by vapor or sputter deposition techniques, as discussed beginning at about column 3, line 30 through line 65. The support sheet with the attached dressing is suspended in a vacuum vessel and all of the air is removed. The material is vaporized by heating to its melting point with an electron beam.
U.S. Pat. No. 4,738,849, teaches a composite wound dressing with separate layers which are subjected to what appears to be a liquid treatment followed by freeze drying.
U.S. Pat. No. 4,997,425, shows a wound dressing which in one embodiment claims to release the antimicrobial little by little (column 4, lines 48-53). As described beginning at column 5, lines 12-20, the antimicrobial solution is poured into a vessel and then subjected to quenching and freezing under vacuum so that a sheet-molded porous layer is produced.
U.S. Pat. No. 5,445,604, shows a wound dressing of multiply layers which includes a “net” in which the strands and junctures are formed integrally during manufacture (column 3, lines 8-10). The reference discusses the use of a hydrophilic polyurethane foam together with a silver salt, e.g., beginning at about column 9, lines 26-65 which discusses “incorporation into the proto-foam prior to polymerization.”
U.S. Patent Publication No. 2002/0168400 shows a resin foam wound dressing in which the resin layer has a collagen layer dispersed over the foam layer, following by freeze drying.
U.S. Patent Publication No. 2005/0124724, shows a polymer composition which has a bio-active agent “distributed therein.” This reference discusses dispersing metal oxide particles “within the hydrophilic polymer” (paragraph 0052).
Despite the advances in the art represented by references of the above type, a need continues to exist for an improved wound dressing-treatment of the type under consideration.